This invention relates to ionization current detector device for detecting an ionization current which flows across a gap situated within cylinders of an internal combustion engine and which is indicative of the combustion states within the cylinders.
There are various methods for detecting the combustion states in the cylinders of an internal combustion engine. One among them is the method according to which the ionization current flowing across a gap situated within cylinders of an engine is detected. The principle of operation of ionization current detector devices for detecting the combustion states of an engine is well known: During the combustion period of the cylinders of the engine, a great amount of gaseous ions is generated within the cylinders due to the chemical reactions involved in the combustion. The concentration of ions, which is indicative of the state of the combustion within the cylinders of the engine, can be detected by means of the ionization current flowing across air gaps situated within the cylinders. Thus, by disposing an air gap within a cylinder of the engine and then applying a voltage thereacross, the combustion state can be detected by means of the ionization current which flows across the gap.
FIG. 1 shows the organization of a conventional ionization current detector device for detecting the combustion state of an internal combustion engine. In the figure, reference numeral 1 represents a cylinder of the engine, and the output terminal of the secondary side of an ignition coil 2 is coupled to a terminal of a spark plug 3 at the top of the cylinder 1, wherein the other terminal of the spark plug 3 is grounded. Thus, when the current supplied to the primary side coil of the ignition coil 2 is turned off, a high voltage of negative polarity is induced at the output terminal of the secondary side of the ignition coil 2, so that a spark is struck across the gap of the spark plug 3 to ignite and combust the air-fuel mixture within the cylinder 1. On the other hand, an ionization current detector plug 4 has an air gap disposed within the cylinder 1 for detecting the ionization current which flows thereacross; one of the two terminals of the detector plug 4 is grounded, while the other terminal is coupled to the negative terminal of a DC voltage source 5, whose positive terminal is coupled to ground via a resistor 6. The voltage developed across the detector output terminals 7 coupled across the two ends of the resistor 6 is proportional to the ionization current which flows across the gap of the detector plug 4.
Thus, the detector device of FIG. 1 can detect the ionization current indicative of the combustion state as follows. When the cylinder 1 is not in the combustion state, the amount of ions in the cylinder 1 is negligible and hence the magnitude of current which flows across the gap of the detector plug 4 is also negligible. Thus, no appreciable voltage is developed across the detector output terminals 7. On the other hand, when the air-fuel mixture within the cylinder 1 is combusted and the concentration of ions within the cylinder 1 increases, the electrical resistance across the gap of the detector plug 4 is reduced. Thus, ionization current flows across the gap of the detector plug 4 and through the voltage source 5 and the resistor 6. The voltage drop across the resistor 6, which is proportional to the ionization current flowing across the gap of the detector plug 4, is detected at the terminals 7.
The above ionization current detector device, however, has the following disadvantage. Namely, if the device comprises only one ionization current detector plug 4, the device is only capable of detecting the combustion state of one cylinder; the detector is incapable of detecting the combustion states of the respective cylinders of a multicylinder engine. If the respective combustion states of the cylinders of a multicylinder engine are to be detected, a detector plug unit must be disposed in every one of the cylinder, which complicates the organization of the detector and increases the cost thereof.
FIG. 2 shows the organization of another conventional ionization current detector device for an internal combustion engine which comprises four cylinders. In the figure, the central terminal of the distributor 9 is coupled to the output terminal of the secondary side of the ignition coil 2, while the peripheral terminals of the distributor 9 are coupled to the spark plugs 3 of the respective cylinders of the internal combustion engine. The ionization current detector device is coupled across a peripheral terminal of the distributor 9 and the ground so as to detect the ionization current which flows across a gap of the spark plugs 3. Namely, there is coupled, across a peripheral terminal of the distributor 9 and the ground, a series circuit consisting of: a diode 8 whose anode is coupled to the peripheral terminal of the distributor 9; a high voltage source 5 whose negative terminal is coupled to the cathode of the diode 8; and a resistor 6 of the current detection circuit 6a, across the two terminals of which is developed a voltage proportional to the ionization current flowing across the gap of a spark plug 3.
The method of operation of the detector of FIG. 2 is similar to that of the detector of FIG. 1, except that the ionization current flows across the gap of a spark plug instead of the narrow gap of a separate detector plug. It is noted in this connection that in the case of the detector of FIG. 2, a voltage source 5 providing a high voltage is required, and that the detector circuit 6a is coupled to a high voltage lead of this high voltage source 5. Thus, in addition to the need for the provision of a high voltage source 5, the ionization current detector device of FIG. 2 has the following disadvantage. Namely, for the purpose of grounding a terminal of the detector resistor 6, which is coupled to the high voltage source 5, at a location which is removed from the distributor 9, the disposition of a long high voltage cord is required; this disposition of a long high voltage cord results in the generation of electromagnetic noises.